Testolactone, an aromatase inhibitor, is currently under evaluation for use in the treatment of infertility in males. However, there are no published data regarding the pharmacokinetics of testolactone. We have developed an HPLC assay that is specific, precise, and sensitive enough for monitoring urine and plasma levels of testolactone (sensitivity of 20 mg/ml). The assay uses an internal standard (testosterone) for quantitation and involves monitoring ultraviolet absorbance of testolactone following a methylene chloride extraction step and work-up. Plasma and urine samples were obtained from a male patient receiving 500 mg testolactone 4 times daily as an oral capsule. Samples will be analyzed from 7 patients in an attempt to define the disposition profile, urinary excretion and plasma half-life in males. Preliminary results from one patient show a plasma half-life of about 1.4 hours, with peak absorption occurring at 2 hours after an oral dose. An unknown peak was observed in the chromtograms of plasma and urine identification of this substance, a possible metabolite of testolactone, are in progress.